In a conventional radio frequency signal receiver for FSK modulation, as shown in FIG. 1a, direct frequency conversion can be carried out in two distinct quadrature branches to obtain baseband signals. Each branch includes a mixer 4, 5 for performing the frequency conversion with oscillating signals supplied by a local oscillator 7. In a first branch, a first high frequency mixer 4 mixes the FSK signals picked up by the antenna 2 and amplified by the low noise amplifier (“LNA”) 3 of the receiver, with in-phase oscillating signals SI to provide intermediate in-phase signals IINT. In a second branch, a second high frequency mixer 5 mixes the FSK signals picked up by antenna 2 and amplified by the LNA 3, with quadrature oscillating signals SQ to provide intermediate quadrature signals QINT. These quadrature oscillating signals are obtained via a 90° phase shifter 6 connected to local oscillator 7. The intermediate signals IINT and QINT are then each filtered in a respective low-pass filter 8 and 9 to provide filtered signals. The filtered signals then each pass through a respective limiter 10 and 11 prior to data demodulation in the conventional demodulator 12, which provides data signals DOUT. The two intermediate in-phase signals IINT and quadrature signals QINT are necessary for the demodulation stage to be able to identify the sign of the incoming FSK signal frequency drift and to analyse the incoming FSK signal data.
According to another variant of a conventional FSK radio frequency signal receiver shown in FIG. 1b, a 90° phase shifter 6 may be provided in one of the branches prior to frequency conversion. The FSK radio frequency signals are mixed, firstly, in the first high frequency mixer 4 via oscillating signals SI provided by local oscillator 7, so as to provide intermediate in-phase signals IINT. The FSK radio frequency signals phase-shifted by 90° by phase shifter 6 are then mixed in the second high frequency mixer 5 via the same oscillating signals SI from local oscillator 7, so as to provide the intermediate quadrature signals QINT.
According to the first and second variants set out above with reference to FIGS. 1a and 1b, receiver 1 may be capable of picking up conventional FSK radio frequency signals, which may have a carrier frequency f0 of around 2.45 GHz. A data modulation frequency drift or deviation of +Δf or −Δf around f0 may be around ±250 kHz, or lower. Since high frequency direct conversion is carried out in the two mixers 4 and 5, followed by filtering in the two low-pass filters and amplitude limiting in two limiters, the receiver consumes a high level of electric power, which is a drawback.
To avoid the use of two mixers during the high frequency direct conversion of the incoming radio frequency signals, reference may be made to U.S. Pat. No. 5,293,408, which discloses an FSK data signal receiver. This receiver has a single mixer for the direct conversion of the incoming FSK signals into baseband signals. To achieve this, according to a first variant, oscillating signals from a local oscillator are supplied to the single mixer via a phase control circuit, which acts alternately during phase switching, like a 90° phase shifter. The phase control circuit supplies alternately over time in-phase oscillating signals and quadrature oscillating signals to the single mixer to convert the frequency of the FSK radio frequency signals picked up by the receiver antenna. The intermediate signals supplied by the mixer are thus a series of alternate intermediate in-phase signals and intermediate quadrature signals. These intermediate signals are filtered in a low-pass filter prior to a data demodulation operation.
According to a second variant of the FSK radio frequency signal receiver, the phase control circuit, which acts alternately during phase switching as a 90° phase shifter, is arranged between the receiver antenna and the single mixer. This phase control circuit supplies alternately over time in-phase FSK radio frequency signals and quadrature FSK radio frequency signals to the mixer. The local oscillator directly supplies oscillating signals to the mixer so that it provides alternate intermediate in-phase and quadrature signals. These intermediate signals are also filtered by a low-pass filter prior to a data demodulation operation.
In the FSK signal receiver of U.S. Pat. No. 5,293,408, switching between the in-phase signals and quadrature signals by the phase control circuit is carried out very abruptly. This leads to high frequencies. In these conditions, because very rapid switching is required, a low-pass filter with a very broad bandwidth is needed after the single mixer, which is a drawback. This also means that the FSK signal receiver has low practical sensitivity and poor reception channel efficiency, but high electric power consumption. This receiver can only be used for controlling equipment in a precise place, but not in a communication universe with several transmission and reception channels. Moreover, the phase control circuit for changing from in-phase signals to quadrature signals and vice versa is directly controlled on the basis of first baseband signals at the low-pass filter output. Switching with very steep flanks occurs from one phase to another, which requires the use of lag components. Thus all the spurious frequencies on the central frequency pass through the filter to maintain rapid switching between the phases. These spurious frequencies may even be higher than the frequency of the incoming radio frequency signals, which leads to high power consumption in the receiver input stage.
U.S. Pat. No. 6,038,268 disclosing an FSK radio frequency signal receiver may also be cited. This receiver also uses a single mixer for converting the frequency of the radio frequency signals into baseband signals. A phase control circuit is provided at the local oscillator output to supply in-phase or quadrature signals to the mixer. A pulse control generator is provided for clocking the phase control circuit at the local oscillator output. As for the preceding document, the first low-pass filter at the mixer output has a very high bandwidth, given that phase switching must also be very rapid. This is a drawback, since second low-pass filters with a narrow bandwidth must also be provided before the data demodulation phase to remove all of the spurious frequencies, which have not been filtered out by the first low-pass filter. High power consumption in the input stage, notably the phase control circuit, is thus observed, and additional components are provided between the frequency conversion and data demodulation, which is another drawback.